1. Field of the Invention
This invention relates to an improvement of a laser processing apparatus for changing a Gaussian power distribution beam of a high power laser, for example, a YAG laser, CO2 laser, excimer lasers etc., into a uniform tophat power distribution beam which has a constant power density in a definite area, and multiplying the tophat distribution beam into a plurality of identical beams by a DOE (diffraction optical element) and heat-treating, annealing, hole-boring, welding or polishing many points on an object material.
This application claims the priority of Japanese Patent Application No.2003-22967 filed on Jan. 31, 2003, which is incorporated herein by reference.
Beams generated by gas lasers or solid state lasers have an axial-symmetric, nonuniform power distribution which has a higher power density at a center and a lower power density at a periphery. The nonuniform power distribution of the gas laser or solid state laser beams is akin to a Gaussian distribution. Thus, we approximate a power distribution of original laser beams as a Gaussian distribution. If object laser processing requires a uniform power distribution beam, the original Gaussian beam should be converted into the uniform power distribution beam by expanding a central part and shrinking a peripheral part of the original Gaussian beam by some optical apparatus.
The Gaussian/uniform distribution conversion is achieved, for example, by a set of two lenses consisting of an intensity-modifying lens L1 and a phase-adjusting lens L2. The first intensity-modifying lens L1 has a central concave part for expanding a central portion of an input Gaussian beam and a peripheral circular convex portion for shrinking a peripheral portion of the Gaussian beam. The intensity-modifying lens L1 produces a uniform power distribution beam from the Gaussian beam. The reformed beam includes imparallel rays with irregular phases. The phase-adjusting lens L2 which has a curvature surface for compensating the L1 lens restores the common phase and parallelism to the beam. A beam output from the L2 lens is a set of phase-aligning, parallel rays.
The conversion from a Gaussian power laser beam to a uniform power beam is called “homogenizing”. An apparatus of the Gaussian/uniform conversion is called a “homogenizer” or “homogenizing optics”. A word “tophat” is used for representing a uniform power distribution in a definite area with a discontinuous boundary. Another word “supergaussian” is used for representing a quasi-uniform power distribution in a definite area with a continuous boundary in this description. Equalizing a sum of amplitudes of light rays at a point is not enough for making a uniform power (tophat) beam. Light has amplitude and a phase. If rays are out of phase, a beam which is once converted to be a tophat beam at a point, soon degenerates to be a non-uniform beam. Thus, “homogenize” denotes not only to equalize the power density (or amplitude) but also to equalize phases and to keep rays to be parallel. Phases are often forgotten. But phases are as important as intensity for light. Like a plane wave, a homogenized tophat beam (f=1 at r≦a, f=0 at r>a) should have properties of uniform power, parallelism, common phase (in-phase) and monochromacity.
2. Description of Related Art
Many contrivances have been proposed for obtaining a uniform power (tophat) distribution beam from a Gaussian beam.
(1) Beam Splitting/Superposing Method
Chalidescopes or segment mirrors have two-dimensional lattice cells which divide a Gaussian laser beam into M×M lattice beams and superpose the M×M divided beams at a single cell object point. Since all the cell beams are randomly added in the single point and in-cell fluctuation is cancelled, the power distribution should be nearly uniform. Since many incoherent beams are superposed, strong interference occurs among beams. Phases and directions of rays are randomly perturbed. If the superposed beams just hit an object at the single point, the power uniformity would be maintained. When the once-uniformed (homogenized) beam further propagates in space, random phases and random directions deprive the beam of uniformity. The beam splitting/superposing method is unsatisfactory with regard to isophase, parallelism and perpetual uniformity which are important conditions of homogenized beams.
(2) A Two Lens Method Having a Set of an Intensity-modifying Lens and a Phase-adjusting Lens
{circle around (1)} U.S. Pat. No. 3,476,463(Justin L. Kreuzer) proposed homogenizing optics including an intensity-modifying aspherical lens L1 with a concave center and a convex periphery and a phase-adjusting aspherical lens L2 which has a surface complementary to the intensity-modifying lens L1. The intensity-modifying lens L1 expands central strong rays of a Gaussian beam by the concave center and concentrates peripheral weak rays into a uniform power distribution by the convex portion. At the forefront of the phase-adjusting lens L2, energy density of rays per unit area is uniform. Although the intensity distribution is uniform, phases and directions are irregular for the rays. The phase-adjusting lens L2 restores a common plane-wave phase and parallelism of the rays. The rays after the phase-adjusting lens are parallel, coherent (common phase), and uniform in a power density.
The phase-adjusting lens L2 is convex at a center and plane at a periphery. L2 has a surface of thickness distribution complementary to the intensity-modifying lens L1. The beam after passing the phase-adjusting lens L2 is a coherent, parallel, uniform power distribution beam. Namely, all individual rays are parallel, in-phase (coherent), monochromatic and isopower (uniform density of power). Parallelism and coherency ensure a constant energy density and a definite section of the propagating beam along an overall light path. It is an ideal tophat beam.
{circle around (1)} obtained thickness distribution functions of the rear surface of the intensity-adjusting lens L1 and the front surface of the phase-adjusting lens L2 for realizing a tophat beam by analytical calculations without approximation. If the intensity distribution of a laser beam is known, full surface shapes of the lenses L1 and L2 can be determined by analytical calculation. {circle around (1)} was an excellent work for tophat optics. {circle around (1)} can make a tophat beam with a uniform energy density within a definite area. But there were still some drawbacks in {circle around (1)}. It cannot enlarge or reduce a size of a tophat beam. The size of the final tophat beam is uniquely determined by the lenses L1 and L2. Another drawback exists in the tophat itself. A tophat function (two-dimensional step function) has sharp discontinuity at edges. The tophat function is indifferentiatable at the edges. Succeeding propagation perturbs ray phases and ray directions, which degenerates a tophat power distribution.
{circle around (2)} Japanese Patent Laying Open No.10-153750, “LASER BEAM SHAPING OPTICAL PARTS” proposed a two-lens system which makes a tophat power distribution beam from a Gaussian beam and enables a tophat image to enlarge or reduce at any magnification/reduction ratio. The system lays an intensity-modifying lens in front and a phase-adjusting lens at the back similar to {circle around (1)}. Probability of magnification/reduction demanded {circle around (2)} to sacrifice parallelism and coherence (isophase) of a final beam. Rays in the beam after the phase-adjusting lens are not parallel. Ideal surface shape (thickness distribution) functions can analytically calculate no more.
Then, {circle around (2)} obtained approximate solutions of lens shape functions by assuming trial lens thickness functions L1(r) and L2(r), calculating amplitudes and phases at surfaces of two lenses by waveoptical manner, examining whether the amplitude and the phase at the rear surface of the second (phase-adjusting) lens are constants, modifying the trial thickness functions for reducing amplitude and phase discrepancies and repeating similar steps many times. Iteration produces approximate solutions of lens shape functions which gives a parallel, isophase(coherent) tophat beam at the rear surface of the phase-adjusting lens. Unlike geometric optics, waveoptic calculation is unidirectional which prohibits reverse calculation seeking for a favorable lens shape functions from final condition of parallelism, isophase of the beam. Iterations of calculations and examinations lead to an adequate set of solutions. Unidirectional waveoptical calculations make plenty of candidate solutions. An optimum set of solutions should be chosen from many candidate solutions by a predetermined criterion.
Waveoptical iterations require a vast amount of calculations. Use of high performance computers can minimize the time of calculations. The reason why many solutions appear in {circle around (2)} is because the phase-adjusting lens cannot remedy wave components perfectly in phase unlike {circle around (1)}. {circle around (1)} required a perfect isophase solution which ensured uniqueness of a solution. Since {circle around (2)} abandons the isophase and parallelism requirements, many candidate solutions appear in {circle around (1)}.
The reason why many solutions are generated is that the calculations are directed from assumed lens thickness distribution to intensity distribution on the rear surface of the lens. Plenty of solutions are yielded in accordance with the assumption of the lens shape. {circle around (1)} is an analytical exact method. {circle around (2)} is an empirical method based on computers. Both try to produce tophat beams by an assembly of an intensity-modifying lens and a phase-adjusting lens. {circle around (1)} can obtain a unique solution. But fabrication of the lens is not easy, since the object lens is an aspherical lens.
{circle around (3)} Fred M. Dickey, Scott C. Holswade, “Gaussian laser beam profile shaping”, Optical Engineering, Vol.35, No.11, November 1996, p3285-3295.
This document proposed a method of varying a Gaussian beam into a uniform power beam in a definite area by a set of two or three lenses. {circle around (3)} alleged that some sets of lenses can make a tophat beam from a Gaussian one.
{circle around (4)} Frank Wyrowski, “Diffractive optical elements: iterative calculation of quantized, blazed phase structures”, J. Opt. Soc. Am. A, Vol.7, No.6, June 1990, p961-969.
This document proposed a contrivance of diffracting a laser beam by a DOE for making a series of letters, for example, “UNI ESSEN FRG” on an image plane.
{circle around (5)} Japanese Patent Laying Open No.2002-202414, “BEAM TRANSDUCER ELEMENT, ILLUMINATION OPTICAL SYSTEM USING THE BEAM TRANSDUCER ELEMENT, ALIGNER, LASER PROCESSING MACHINE AND PROJECTION DEVICE”.
This document proposed optics of converging a Gaussian beam to a uniform (tophat) beam by two holographic elements.
{circle around (6)} Japanese Patent Laying Open No.9-61610, “BINARY OPTICS, AND LIGHT CONVERGENCE OPTICAL SYSTEM AND LASER MACHINING DEVICE USING BINARY OPTICS”.
This document suggested a contrivance of shaping a Gaussian beam to a uniform distribution beam (tophat) by diffractive optical elements.
{circle around (7)} U.S. Pat. No. 6,433,301 (Dunsky et al.), “BEAM SHAPING AND PROJECTION IMAGING WITH SOLID STATE UV GAUSSIAN BEAM TO FORM VIAS”.
This patent converts a Gaussian beam into a tophat beam by a DOE, converges the tophat beam and irradiates objects with the converged tophat beam for boring or other processing. The tophat beam is guided to the objects via a converging lens for heightening power density.
The above laser processing techniques aim at making a tophat (uniform power) beam from a Gaussian laser beam by DOEs or lenses, separating one tophat beam into a plurality of tophat beams, irradiating multipoints on an object with the plural tophat beams, and cutting, boring, annealing or welding the object at the multipoints. It is still insufficient to produce a tophat beam at one time or at a spot. Beams should be maintained to be the tophat power distribution at all times and at all positions. The cited references do not always succeed in keeping uniform power distribution of beams throughout the light paths.
A homogenizer lens or a homogenizer DOE converts a Gaussian beam into a tophat beam at a predetermined spot. But the beam further propagates in space after passing a homogenized spot. Spatial propagation perturbs phases and directions of rays, unless the homogenized beam is fully a monochromatic, coherent, parallel beam.
The aforedescribed {circle around (2)}, which is based upon waveoptical calculations of trial and error, can produce many approximate solutions but cannot obtain a decisive solution. Although a uniform density beam is once established at a point by a DOE based upon the approximate solution, the uniform power density profile degenerates in propagation. Since the non-uniform beam is divided by a DOE, divided beams are degraded into non-uniform branched beams.
The reason is that the intensity distribution at the point has too sharp edges, the inventor of the present invention supposes. And the sharp intensity discontinuity at the edges may cause diffraction after the point, so that the intensity is in disorder.
So, the inventor adopts a supergaussian intensity distribution with gradual edges as a distribution of an object beam, instead of the tophat distribution (f=1 at r≦a, f=0 at r>a). A Gaussian function has r2, which is in proportion to the square of a radius r, in exp( ), but a supergaussian function is in proportion to the nth power of a radius r. The supergaussian distribution is exp{−2(r/a)n}.
N is the order number of the supergaussian function. When n=2, it is equal to a Gaussian function. When n is indefinitely large, it is a tophat function. A definite number of supergaussian functions ensures continuity and differentiatablity at edges. There is a big abyss between a definite number and an indefinite large number. A tophat beam soon loses a uniform beam shape by propagation. The inventor hit a hint of smoothing edges by employing a definite number (n=20-100) of the supergaussian function for a beam shape for avoiding the beam shape degradation in propagation.
The inventor gives an improvement of replacing a uniform, tophat beam by a quasi-uniform supergaussian beam having the same size. The sectional shapes are arbitrary. The present invention produces a cylindrical supergaussian beam instead of a round tophat beam. Alternatively, the present invention makes a rectangular supergaussian beam instead of a rectangular tophat beam. A 2a×2b supergaussian beam can be represented by exp{−2(x/a)m−2(y/b)n}. An ellipsoidal supergaussian can be represented by exp[−2{(x/a)2+(y/b)2}n]. Supergaussian order numbers are m and n. A sectional shape is 2a×2b. Regions within r<a (round beam) or within |x|<2a, |y|<2b (rectangle beam) are called a uniform region.
A lens can convert a Gaussian power distribution laser beam into a uniform tophat beam. A spherical shaped lens is incompetent. An aspherical lens should be utilized for reforming a Gaussian distribution laser beam into a uniform distribution tophat beam. Fabrication of such an aspherical lens is not easy. The aspherical lens is a high cost optical part, since it takes long time for making the aspherical lens.
A lens homogenizer can be made by an aspherical lens. A lens has smoothly curving surfaces. The height of surfaces changes continuously and smoothly as a continuous function of a radius coordinate “r”. What a lens can do can be done by a DOE. The lens for homogenizing can be surely replaced by a DOE. There is possibility of a “homogenizing DOE”.
DOEs are classified into reflection type DOEs and penetration type DOEs. Principles are common. Here, having an analogy to a lens, the penetration type DOEs are considered. This invention can be built with the reflection type DOEs, of course. A penetration type DOE is an assembly of pixels aligning in x- and y-directions. A pixel is a fundamental square cell having a definite size. A pixel has a free parameter which is a height or a thickness. The thicknesses (or heights) are quantized into discrete 16 values or 32 values.
For example, convex lenses or concave lenses can be replaced by Fresnel lenses having a plenty of concentric circular protrusions with a quasi-uniform thickness. A Fresnel lens is inherently an analog optical device. Heights and diameters of the protrusions are continuous variables in the Fresnel lens. A Fresnel lens can be reduced to a DOE (diffraction optical element) by replacing analog variables by digital ones and continuous areas by discrete pixels. Namely, a convex lens or a concave lens can be replaced by an identical DOE. Similarly, a homogenizing lens can be also replaced by a homogenizing DOE.
Instead of an aspherical lens, a special DOE can homogenize a Gaussian laser beam to a uniform power (tophat) beam. Furthermore, another special DOE would be able to homogenize a Gaussian beam into a supergaussian beam (n is several tens).
The present invention has interest in an optical system of homogenizing a laser Gaussian beam into a supergaussian beam with a DOE. Not a homogenizing DOE but a DOE homogenizing optical system is an object of the present invention. Thus, an actual structure of the homogenizing DOE is not described in detail in the specification.
Assuming that such a homogenizing DOE could be made, probable problems are now clarified. Unlike a lens homogenizer, a DOE homogenizer builds a quasi-uniform distribution by dividing a beam into a plurality of rays, diffracting rays, assembling rays and superposing rays in accordance with some rule. In the case of a lens homogenizer, the quasi-uniform distribution plane exists just at a front surface of an phase-adjusting lens. Similarly, a DOE homogenizer builds a quasi-uniform distribution plane at a point distanced from the DOE by a certain length. The quasi-uniform plane is called a “reference plane”.
The homogenized beam is then divided into a plurality of branched beams dispersed lengthwise and crosswise by a diverging DOE (diffractive optical element). The branched beams are converged by a lens and irradiated to many object spots aligning at definite intervals on an image plane (object plane).
Such an optical system consists of a homogenizer DOE, a diverging DOE, a lens and an object (an object plane).
A beam can be divided into an arbitrary number of beams by a diverging DOE. If the division number were large, the size of a single beam projected on an image plane would be too narrow, which would obscure details of the beam. Then three to five divided beams have been made by a diverging DOE for examining power intensity distribution in the beam spots in detail. As a result, power density fluctuates in three to five homogenized beams spatially which should have uniform intensity distribution. Such a large power distribution would degrade the resultant divided beams as a tophat (or supergaussian) beam.
What is the reason of causing the big fluctuation? A homogenizing DOE makes a uniform power density beam at the reference position. Noise rays occur at peripheries of the beam. Noise is caused by unnecessary diffraction by the DOE. The homogenizer DOE generates various order of diffraction rays. The noisy diffraction rays are also diffracted further by the diverging DOE and are projected onto the image plane. Interference between noise and signals causes power fluctuation of the once-homogenized beams.
The consideration clarifies the reason of inducing the fluctuation of the branched beams.
Then, the inventor inserted an aperture mask at the reference position (focal point of the DOE) for killing noise. The aperture mask is a plate having a window of a size (diameter 2c or sides 2c×2d). The inventor tried to examine an effect of a narrow aperture mask having a window narrower than the beam size, since he thought the edges of the beam induced the fluctuation. For a round beam (2a), a round window (2c) smaller than the beam was tentatively perforated (2a>2c) on the aperture mask. For a square beam (2a×2b), a square window (2c×2d) smaller than the beam was tentatively perforated on the mask(2a>2c, 2b>2d).
The reason why the aperture mask is placed at a reference position is that the beam is homogenized at the reference position. The reference position is a focal point of the homogenizer DOE. The lens produces an image of the aperture mask on the image plane.
The aperture mask eliminates some portion of the beam or the noise. The window of the aperture mask determined the shape of the beam.
The homogenizer optics system makes a uniform power density beam by a homogenizer DOE at a reference position, annihilates annular edge parts of the beam by an aperture mask at the reference position, divides the edge-cut beam into a plurality of beams by a diverging DOE, converges the divided beam by a lens and irradiates a plurality of object spots with the divided beams for heating and lightening processing. The lens can be an ordinary ftanθ lens or a sophisticated fsinθ lens. The fsinθ lens enables plural separated beams to produce irradiated spots aligning at a definite period.
The homogenizer optic system can simultaneously achieve boring, welding, annealing, cutting or do other optical/thermal processing of many spots or many lines in parallel. In addition to two-dimensionally distribution round spots, the homogenizer system can accomplish similar processing on many parallel lines. Lines are a limit of increasing a vertical/horizontal ratio of a rectangle. Lines can be treated in a manner similar to spots. In the case of linear objects, a homogenizer DOE of making a homogenized rectangle beam of a high ratio a/b and a diverging DOE of inducing one-dimensional diffraction should be employed.
Some kinds of heat/light processing require uniform power distribution on many object points. Equipollence of power is important for ensuring same quality of processing on all the points. Individual object spots require spatial uniformity of power density without fluctuation. Such an elaborate processing requires two different steps of uniformity. One is allover uniformity among plural object points. The other is individual uniformity within individual object spots.
The abovementioned optics simply consists of a homogenizer DOE 3, an aperture mask 4, a diverging DOE 5, a lens 6 and an object (image plane) 7, as shown in FIG. 1. A quasi-uniform, supergaussian beam is produced at the aperture mask 4. The diverging DOE 5 divides the beam into a plurality of branched beams. The branched beams are converged and irradiated onto many object spots on the image plane 7. The lens 6 produces an image of the aperture mask 4 on the image plane 7.
The above mulitipoint lump homogenizer system has still a problem to be solved. The problem is now clarified.
The beam should have quasi-uniform power distribution within a radius a which is represented by the supergaussian function exp{−2(r/a)n}. If a narrow aperture mask of a 2c diameter (2c<2a) is placed at the focus, the aperture mask eliminates edge parts (c<r<a) of the beam. Intensity vehemently fluctuates at the edge (r=a). If the edges are annihilated by the mask, good inner parts remain. When the good parts are diffracted onto an image, good diffracted beams would be obtained.
But, it is not true. The fact is that the edge-eliminated good beam makes separated beams which have large power fluctuation on the image plane. Since the edge parts c<r<a are eliminated, the power uniformity in a beam is heightened at the position of the aperture mask. But the beam propagates further in space. Sharp cut edges induce strenuous diffraction which causes vehement power fluctuation of the branched beams on the image plane and degenerate supergaussian distribution.